1. Technical Field of the Invention
The present invention relates to a servo control apparatus for driving a load, in particular a servo control apparatus that can control the load so that it follows the reference control value even if an abnormality occurs.
2. Prior Art
With a servo control apparatus that drives the transfer mechanism of a press machine, for instance, as the load, even if an abnormality takes place in a component device (servo controller, servo amplifier, servo motor, feedback detector, etc.) of the servo control apparatus, the transfer mechanism must operate in synchronism with the dies until the dies stop; otherwise the dies and the transfer mechanism may collide with each other. Therefore, it is mandatory for the servo control apparatus to continue to control the load even when a component device of the servo control apparatus fails.
One of the technologies known in the prior art to be capable of continuing the servo control of the load even when the servo control apparatus fails is a torque master-slave system that is configured with two sets of servo amplifiers and servo motors to drive the load.
This apparatus, an example of which is shown in FIG. 1, is provided with one master servo control circuit 2, composed of a servo controller 3 that computes the torque command signal so that a load (not illustrated) follows the reference control signal determined by a reference control adjuster 1, a servo amplifier 4 that controls the electric power supplied to a servo motor according to the torque command signal provided by the servo controller 3, the servo motor 5 for driving the load supplied with electric power from the servo amplifier 4, a feedback detector 6 to detect a feedback value from the servo motor 5, an abnormality detector 7 that detects abnormalities in the servo controller 3, servo amplifier 4, servo motor 5 and feedback detector 6 and sends command signals to the reference control adjuster 1 and servo amplifier 4.
The servo controller 3 outputs a torque command signal according to the reference control signal sent from the reference control adjuster 1 and the feedback signal from the feedback detector 6, and the calculated torque command signal is input into the servo amplifier 4. The servo amplifier 4 provides an electrical output to the servo motor 5 so that the servo motor 5 produces the torque required by the torque command signal.
In addition, this apparatus is provided with a slave servo control circuit 8 configured with a servo amplifier 9 with the same configuration and functions as the above-mentioned one, a servo motor 10, and an error detector 11 that detects abnormalities in both the servo amplifier 9 and servo motor 10 and sends a command signal to the reference control adjuster 1 and the servo amplifier 9. Furthermore, this apparatus is configured additionally with a shaft 12 that is driven by servo motors 5 and 10 so that they are mechanically interlocked with each other and drive the load, and a power transmission mechanism 13 for transmitting the rotation of the shaft 12 to the load to drive the load.
Another technology also known in the prior art relates to the servo control apparatus disclosed in the unexamined Japanese patent publication No. 87985, 1990. This apparatus is an AC servo motor drive control circuit 14 wherein AC current is converted to DC current by a single-phase rectifier circuit 16 in a converter unit 15, and a 3-phase AC servo motor 17 is driven and controlled using the DC current. The apparatus is provided also with relays 19, 20 that when a failure occurs, connect two of the three-phase power lines of the AC servo motor 17 to the smoothing capacitor 18, a component device in the converter unit 15. Because these relays 19, 20 connect the AC servo motor 17 to the smoothing capacitor 18, electrostatic energy stored in the smoothing capacitor 18 is supplied to the AC servo motor 17, and produces a stationary magnetic field, thereby stopping the AC servo motor 17.
However, according to the method based on the master-slave torque system shown in FIG. 1, only one servo controller 3 and feedback detector 6 are employed, and the servo amplifiers 4, 9 receive torque command signals from one servo controller 3. Consequently, if an abnormality occurs in the servo controller 3 or the feedback detector 6, both servo amplifiers 4, 9 cannot receive a torque command signal, so in this case, both servo motors 5, 10 go into a free running state.
Therefore, when either the servo controller 3 or the feedback detector 6 fails while the load is being operated, the load must be stopped by either (1) waiting for it to stop naturally due to friction, (2) using the braking force produced by a mechanical brake, or (3) using a dynamic braking system in which the armature of either servo motor 5 or 10 is short circuited,
However, according to method (1), there is the problem that the load continues to run for a long time because the friction force acting on the load driven by the servo motor is normally small. Method (2) has the problem that the distance that the load overruns becomes large since there is a considerable time delay before the mechanical brake is actuated. In method (3), the dynamic brake is turned ON and OFF by mechanically activated contacts, and the time delay before the dynamic brake is operated is so large that the load overruns, and there is the problem that an AC motors cannot be used for servo motors 5 and 10 and only DC motors can be used. Another problem that exists with any of the above (1), (2) and (3) is that after the load enters the free running state, the position and speed of the load can no longer be controlled.
On the other hand, in the AC servo motor drive control circuit 14 shown in FIG. 2, the load can be prevented from overrunning, but there is the problem that the position and speed of the load cannot be controlled. In addition, there is another problem because only one AC servo motor 17 is used, so if the AC servo motor 17 fails, the servo-control function is lost.
The present invention aims at solving the problems mentioned above. An object of the present invention is to provide a servo control apparatus that can continue to control the load so that it follows the reference control value even if an abnormality occurs in any portion of a servo controller, servo amplifier, servo motor, or feedback detector.
To solve the aforementioned problems, the present invention provides a plurality of servo control circuits each of which can singly control a load; each servo control circuit is configured with a servo controller that computes command signals so that the load can follow a reference control value adjusted by a reference control adjuster, a servo amplifier for controlling the electric power supplied to a servo motor according to a command signal from the above-mentioned servo controller, the servo motor that is supplied with electric power from the servo amplifier and drives the load, and a feedback detector that detects the operating condition of the servo motor and feeds the detected condition back to the servo controller; the servo motor in each of the aforementioned servo control circuits is interlocked mechanically with each of the others so as to be capable of driving the one load through a drive transmission mechanism, and if an abnormality takes place in any of the above-mentioned plurality of servo control circuits, the other servo control circuits, which are still normal, can continue to control the load.
When all the servo control circuits are normal, each servo control circuit cooperates with each of the others to control the load so as to follow the reference control signal.
If an abnormality arises in any of a servo controller, servo amplifier, servo motor or feedback detector in any servo control circuit, the power from the servo amplifier to the servo motor in the servo control circuit with the abnormality is interrupted, thus the aforementioned servo motor is made to run freely, thereby normal servo control circuits can continue to control the load so as to follow the reference control signal.
Therefore, because the reference control adjuster is made to generate a reference control signal that stops the load when an abnormality occurs in any of the servo control circuits, the other servo control circuits, which are still normal, continue to control the load so that the load is stopped as it follows the reference control signal.
Among a plurality of sets of servo control circuits, one is defined as the master servo control circuit, and the other servo control circuits are considered to be slaves; a slave servo amplifier is given a command signal from the master servo controller during normal operation, at a location upstream of the servo amplifier in the above-mentioned slave servo control circuit; and a command signal selector is provided and can switch to the command signal input from the slave servo controller when an abnormality occurs in the master servo control circuit; thereby the load can be controlled in master-slave fashion to follow the reference control signal by both the master and slave servo motors when no abnormality appears in any of the servo control circuits on the master or slave sides.
Other objectives and advantages of the present invention are revealed in the following paragraphs referring to the attached drawings.